Electroactive composites include piezoelectric and electrostrictive materials. Piezoelectric and electrostrictive materials are those materials which produce dimensional changes when under the influence of an electrical field. Conversely, when put under stress conditions, piezoelectric materials can develop an electrical field, or in open circuit conditions, a polarization.
Conventional, non-stress-biased electroactive devices exhibit relatively limited mechanical displacements when an electric field is applied. These devices can be generally described as electroactive structures which include at least one piezoelectric or electrostrictive material attached or bonded to at least one other material which can operate as an electrode, support layer, strain amplification material, or the like. The output of such conventional piezoelectric devices is limited by the characteristics of the piezoelectric material, however. Thus, conventional devices of reasonable thickness (i.e. on the order of a few millimeters) in many cases offer mechanical output motion only in the micrometer range.
There is a continuing drive in the industry for devices, such as actuators, that are capable of achieving a greater mechanical displacement for a given voltage. Furthermore, there is a continuing desire in the industry for mechanical devices that can achieve mechanical displacements equivalent to displacement of currently known devices, but using less power.
Commercially available stress-biased piezoelectric and electrostrictive devices provide enhanced displacement and load bearing capabilities as compared to conventional devices and other flextensional devices. These stress-biased devices, as with conventional devices, may be rectangular, square, or circular, but in general they consist of a domed composite structure that results from processing and manufacturing conditions employed in their construction.
U.S. Pat. No. 6,060,811 to Fox, et al. is directed to the mounting of a support layer to induce a stress within an electroactive material in production of a stress-biased electroactive device. The resulting device may be mounted in a variety of configurations for different sensing and actuator applications. One device disclosed by Fox, et al. is sometimes referred to as “THUNDER”®, which is an acronym for a “thin unimorph driver”, and is a trademark of the Face International Corporation.
U.S. Pat. No. 5,471,721 to Haertling is directed to methods for making monolithic pre-stressed ceramic devices which are known commercially as “RAINBOW®” devices, an acronym for a “reduced and internally biased oxide wafer”. Haertling discloses monolithic, internally asymmetrically stress-biased electrically active ceramic devices and methods for making such devices. The patent discloses the fabrication of a ceramic element having first and second opposing surfaces. The first surface is chemically reduced by heat treatment in a reducing atmosphere, to produce an internally asymmetrically stress-biased ceramic element.
In both the RAINBOW® and THUNDER® devices, composite structures are formed which incorporate a piezoelectric or an electrostrictive layer bonded to a metal layer (as in the THUNDER® device), a cermet layer (as in the RAINBOW® device), or some other suitable substrate layer to form a stress-biased electroactive composite. The specifics of the fabrication procedures that have been employed in the past for these two devices differ, however each procedure results in a stress-biased, domed structure which is formed as a result of the processing conditions employed. As a result of the manufacturing process, stresses of high magnitude develop within the piezoelectric layer. These stresses, notably the tensile stresses in the surface region of the device, have been reported to contribute to the greater observed electromechanical response.
There is a continuing need in the industry for electroactive devices that produce a greater amount of mechanical displacement for a given voltage. Furthermore, electroactive devices that can produce equivalent mechanical displacements using less voltage are also sought after. In some applications, the power consumption of the electroactive devices is extremely critical, such as, for example, space applications and underwater propulsion. In other applications, power consumption is not critical but the amount of mechanical displacement can be very important. There is a continuing need to improve the performance of electroactive devices to increase the efficiency of actuators, pumps, switches, sensors, variable focus lenses, strain gauges, and other components in which such electroactive devices can be employed.